Empty GroundwaterFlowProcess and its Base classes.
[PL] Add ProcessLib to CMake files.
Add ProcessLib to doxygen inputs.
[PL] Add virtual initialize().
[PL] Add number of node variables.

Element variables are not yet used.
[PL] Add destructor for GroundwaterFlowProcess.
Adjusted GUI for removed FEMCondition classes
[PL] Include class Process into ProjectData.
[Tests] Removed TestBoostXmlCndInterface.

The test tested class BoostXmlCndInterface, that is removed previously.
[CLI] Vector for storing ProcessVariables.
[ProjectData] Make attribute name readable/consistent.
[ProjectData] Add configuration of project to ProjectData.
[ProjectData] Path argument for ProjectData constructor.
[ProjectData] Functionality to read geometry.
[ProjectData] Functionality to read mesh.
[ProjectData] Change deallocation order.

The processes first, because they depend on meshes; then the meshes.
[ProjectData] Add findMeshByName() as private.

This returns an iterator to the found mesh.
Two versions are required for the modifying and non-modifying algorithms.
[ProjectData] Rewrite getMesh(), and meshExists().
[ProjectData] Correct implementation of removeMesh().
[PL] Add mesh subset to initialize().